This invention relates to a light-emitting diode (LED) and a fabrication method thereof, and more particularly to an edge-emitting LED (EELED) having a stripe-geometry light-emitting region disposed in a V-shaped channel of a semiconductor substrate, which emits light from an edge of the light-emitting region.
Although LED's are characterized by a small optical output, they are superior to semiconductor lasers in temperature stability in that they operate reliably over a wide temperature range, and have other excellent features.
An EELED having a stripe-geometry light-emitting region is particularly suited as a light source for optical communications, because it permits control of the transverse mode parallel to the junction can be coupled to a single-mode optical fiber with high efficiency.
An oblique view of the structure of a typical prior-art stripe-geometry EELED is shown in FIG. 1. The reference numerals in FIG. 1 will be used in the descriptions of other drawings showing identical components, and duplication of the explanations of these components will be avoided.
In this example, a first electrode 1 is provided on the lower surface of a p-InP semiconductor substrate 2. On the upper surface of the semiconductor substrate 2 is formed a buffer layer consisting of a p-InP layer 2a, and on the layer 2a is grown a first blocking layer consisting of an n-InP layer 3, over which is further grown a second blocking layer consisting of a p-InP layer 4.
On this semiconductor substrate 2 with its grown layers, a V-channel 5 is formed that penetrates from the second blocking layer 4 through the first blocking layer 3 and the buffer layer 2a and reaches the semiconductor substrate 2.
A p-InP layer 6 is formed on the inner surface of the V-channel 5 as a first cladding layer, and a p-InGaAsP layer 7 is formed as an active layer on the first cladding layer 6.
On the portion of the upper surface of the second blocking layer 4 in which the V-channel 5 has not been etched, and on the active layer 7 grown inside the V-channel 5, an n-InP layer 8 is formed as a second cladding layer. An n-InGaAsP layer 9 is further formed as a cap layer on this second cladding layer 8, and a second electrode 10 is formed on this cap layer 9.
In this way a stripe-geometry light-emitting region is created within the V-channel 5 by sandwiching the active layer 7 between the first cladding layer 6 and the second cladding layer 8.
In the structure described above, the drive current of the EELED is supplied from the first electrode 1 to the semiconductor substrate 2, is confined by the p-n junction formed by the first blocking layer 3 and the second blocking layer 4 to the active layer 7 disposed between the first and second cladding layers 6 and 8, and reaches the second electrode 10 through the cap layer 9. A spontaneous emission of light takes place at this time from the active layer 7 (the shaded area in the drawing) which forms the light-emitting region.
An EELED having a stripe-geometry light-emitting region formed in a V-channel, which delivers spontaneously-emitted light, does not differ in principle from a semiconductor laser with a V-channel stripe geometry. A consequent defect of an EELED described above is that at low operating temperatures, or when the drive current is increased to intensify the light output, a stimulated emission will frequently occur in the light-emitting region, so that the EELED, which should produce spontaneously-emitted light, emits laser light.
In prior-art EELED's, attempts have been made to suppress the occurrence of stimulated emission and obtain a spontaneous emission by applying an anti-reflective coating of silicon nitride having a coefficient of reflection of 1% or less to the facet of the semiconductor substrate that forms the emitting edge of the light-emitting region. The attempts are reported for example in "42 km Analog Video Signal Transmission in Single-mode Fibers Using a 1.3 .mu.m Edge-Emitting LED," M. Matsuura et al, The Transactions of the IECL of Japan, Vol. E 69, No. 4, Apr. 1986, pp. 349ff, and in "1.5 .mu.m Tai Tammen Hakkogata Daiodo" (A 1.5 .mu.m Edge-Emitting LED), a paper presented at the "Denshi jouhou tushin gakkai soritu 70 syunen kinen sougou zenkokutaikai syowa 62 nen" (1987 General Meeting of the Institute of Electronics, Information and Communication Engineers of Japan), No. 882, 4-44.
A problem in the prior art described above is the incapability of producing a stable, spontaneous emission of light under a wide variety of operating conditions, because simply applying a anti-reflective coating to the emitting edge of a stripe-geometry light-emitting region does not suppress stimulated emission of light when the EELED is operated at a low temperature or a high optical output level.